1. Field of the Invention:
The present invention relates to inching control systems, and more particularly, to a rear drive inching control system.
2. Description of the Prior Art:
Rough terrain vehicles for performing work functions such as forklifts, must maintain a high engine speed to perform the intended work function while having a precise low speed drive control for slow travel. One means of achieving such low speed drive control is by built-in modulation of the drive line transmission.
An inching system used in a conventional rough terrain vehicle with mechanical front drive and hydraulic rear drive employs a complicated linkage system to control a variable potentiometer which in turn controls a torque motor actuated pilot valve. The pilot valve outputs a differential pressure proportional to the input current. The output pressure controls a forward-reverse valve which in turn controls the drive system. Inching systems of this kind are difficult to maintain and, due to hydraulic fluid leakage through the pilot valve, experience control problems at low engine speeds.
A variety of inching control systems have been proposed. For example, Zeller et al. U.S. Pat. No. 4,006,805 describes an inching control system for a forklift or similar vehicle. The system includes a pump, a directional control valve, forward/reverse clutches, an inching valve, a narrowed orifice and a braking system. The pump draws fluid from a reservoir to supply the directional control valve. Depending on the operator-selected positioning of the directional control valve, fluid is directed toward the forward or reverse clutch. Intermediate the pump and the directional control valve is a bypass inching valve and a narrowed orifice. The fluid normally flows through the inching valve to the directional control valve, bypassing the orifice. When the brake is depressed slightly, the inching valve begins to close, restricting flow therethrough. Fluid then reaches the directional control valve through the narrowed orifice, decreasing the pressure to the forward or reverse clutch. Further depression of the brake will stop the vehicle.
Kitano et al. U.S. Pat. No. 3,948,366 describes inching and regulating valves for clutches in industrial vehicles. The inching valve, which is disposed within the fluid circuit connecting a pump to a fluid pressure type clutch, controls the degree to which the clutch is actuated. When the brake is depressed, fluid pressure from the regulator valve flows to the inching valve to be exhausted. A lower pressure is therefore available to a forward/reverse valve than the pressure between the pump and the regulator valve. A lower pressure is thereby exerted against the clutch so that the transmission is moved from a semi-clutched to a declutched condition.
Conrad U.S. Pat. No. 3,240,371 describes an inching control system in which an inching control valve is fluidly connected to forward/reverse clutches. Fluid power can be transferred to boom actuators at any desired speed while the vehicle can be inched as desired due to the clutching control of the inching valve. The inching valve is actuated when the boom is raised by means of a valve operatively connected to a carriage.
Quayle U.S. Pat. No. 3,339,363 discloses a mechanism for permitting high speed operation of an engine in an industrial vehicle for performing work functions, while controlling the fluid available to a hydraulic motor for achieving a slow inching speed. By depressing an inching pedal a control valve in a valve block moves into a position for diverting fluid from the motor and for preventing fluid from leaving the motor, thereby achieving a braking action to control the speed of the vehicle. At the same time, an accelerator can be depressed to speed the engine to provide hydraulic fluid for the desired function.
Additional speed reduction systems are shown by Schroeder U.S. Pat. No. 2,883,015; Dandro U.S. Pat. No. 3,288,256; Eaton U.S. Pat. No. 3,750,762; Mori U.S. Pat. No. 3,702,648 and Hatcher U.S. Pat. No. 3,779,608.
The prior art inching systems are adapted for use with mechanical drive means and are controlled by conventional torque converter combination transmissions with clutches. It is believed that the steering for inching travel can be more precise and better controlled when a hydraulic rear axle steering system is employed. In drive systems having a mechanically actuated front drive means and a hydraulically actuated rear drive means, the prior art inching systems are not suitable.
It is an object of the present invention to provide a rear drive inching system for use in vehicles having mechanically actuated front drive means and hydraulically actuated rear drive means. It is a further object of the present invention to provide an inching system for such a vehicle which offers consistant and precise operator control throughout the range of engine speeds and which has fewer working parts to permit lower costs and easier maintenance.